Yosemite’s Half Dome saw a picturesque dusting of snow this past week while skiers in Tahoe welcomed a shot of fresh powder. Yet California’s Sierra Nevada is plunging deeper into a warm, dry winter that shows little sign of a turnaround. Mile-high mountainsides were low on snow Monday and alpine skies remained a stubborn summer blue. The forecast called for above-average temperatures and virtually no precipitation through at least the first 10 days of February — the third and final month of the state’s peak wet season.